1. Field of the Invention.
The present invention relates generally to the field of routing switches for optical communications networks. More specifically, the present invention discloses an optical routing switch using symmetric liquid crystal cells to increase switching speed.
2. Statement of the Problem.
Optical routing switches are one of the key components in optical networks. The optical switch market has been growing rapidly to become a multi-billion business in recent years. They are to be used in systems such as network protection, signal add/drop, channel cross-connect, etc. Liquid crystal (LC) optical switching is a new technology that has much higher reliability than current mechanical switch technology. Liquid crystal switches having high reliability and excellent optical performance have been built. The response times of these switches, however, are not very fast.
When voltage is applied to a LC cell, the LC molecules are switched from a direction perpendicular to the electric field to a direction along the field (or from perpendicular to along the field, depending on the sign of the material""s dielectric anisotropy). This is referred to as the xe2x80x9cturn-onxe2x80x9d process. The switching time can be expressed as:   τ  ∝            γ      ⁢              xe2x80x83            ⁢              d        2                            k        1            ⁡              (                                            V              2                        /                          V              0              2                                -          1                )            
where xcex3 is a viscosity coefficient, k1, is the elastic constant, V is the applied voltage, V0 is the threshold voltage of the material and d is the thickness of the liquid crystal layer. xcex3, k1, and V0 are material constants. d depends on the birefringence of the material. Once a LC material is chosen, we are left only being able to make the switching faster by apply a higher voltage V.
After the voltage is removed, the liquid crystal restores its original state. This relaxation process, however, is completely determined by the LC material constants. It is usually much slower than the turn-on process and thus limits the overall switching speed.
FIG. 1 depicts a conventional switching scheme in which the LC cell is switched back and forth in between the voltage-on and voltage-off states. In the voltage-off state, the polarization of the input beam is rotated 90xc2x0 (i.e., the xe2x80x9ccrossxe2x80x9d state). In the voltage-on state, the polarization doesn""t change (i.e., the xe2x80x9cthroughxe2x80x9d state). At room temperature, the typical switching time from voltage-off to voltage-on state is approximately 1 ms and the typical relaxing time back is approximately 70 ms for typical telecommunications applications operating at infrared wavelengths (i.e., around 1550 nm).
To increase the relaxation speed, previous liquid crystal switches usually work at an elevated temperature (e.g., 50xc2x0 C.) because the viscosity coefficient becomes smaller at higher temperatures. This technique helps to increase switching speed to a degree, but is still not very fast. However, this approach has the disadvantages of significantly increasing power consumption and possibly reducing the lifetime of the device.
3. Prior Art.
A scheme of fast liquid crystal operation for liquid crystal displays has been proposed by Thomas J. Haven, xe2x80x9cA Liquid-Crystal Video Stereoscope With High Extinction Ratios, A 28% Transmission State, And One-Hundred-Microsecond Switching,xe2x80x9d SPIE, True 3D Imaging Techniques And Display Technologies, vol. 761, p. 23-26 (1987). In a liquid crystal display having a two-dimension array with a large number of pixels, Haven""s approach requires two LC cells for each pixel, which doubles the cost and weight. This may be why Haven""s scheme has been largely forgotten and has apparently never been commercialized. It should be noted that Haven""s display employs each pair of LC cells only as a shutter mechanism or on/off switch for each pixel. Haven neither teaches nor suggests optical routing. In addition, Haven""s approach is limited to pi-cells.
4. Solution to the Problem. The present invention provides an optical routing switch that uses two liquid crystal cells to produce offsetting rotations of the polarization of the input optical beam to provide optical switching between a plurality of output ports that is both fast and symmetrical (i.e., has the same switching-on and switching-off speed). This approach successfully addresses the problem of asymmetrical switching speeds associated with conventional liquid crystal routing switches, as discussed above. The response time of the present optical switch is in the sub-millisecond range, compared to previous liquid crystal switches that have switching times in excess of 10 milliseconds at room temperature. In addition, the present invention consumes only a minimal amount of power because no heating or temperature control is needed. Compared to mechanical, acousto-optical, thermo-optical and crystal electro-optical switches, the present optical switch has the advantages of high reliability, high speed, low loss, and low cross-talk.
This invention provides an optical routing switch using two liquid crystal cells that can provide offsetting rotations of the polarization of the beam to provide fast, symmetrical switching. The input beam is first polarized and then passes through both liquid crystal cells in series. Both liquid crystal cells have two states (e.g., voltage-off and voltage-on) in which the beam polarization is rotated by predetermined angles (e.g., 0xc2x0 and 90xc2x0), but in opposing rotational directions. A controller selectively rotates the liquid crystal cells through a sequence of steps, beginning with a xe2x80x9cthroughxe2x80x9d state in which both liquid crystal cells are in the first state. The polarization rotations provided by both liquid crystal cells offset one another so the beam polarization remains essentially unchanged. The liquid crystal cells can be rapidly switched to a xe2x80x9ccrossxe2x80x9d state in which only one of the liquid crystal cells is changed to the second state and the polarization of the beam is rotated by a predetermined degree. The liquid crystal cells can then be rapidly switched back to the through state by changing both liquid crystal cells to their second state. Once again, the polarization rotations provided by both liquid crystal cells offset one another so the beam polarization remains essentially unchanged. While remaining in the through state, both liquid crystal cells are allowed to return to the first state to complete the cycle. Both liquid crystal cells relax at the same rate and their offsetting polarization rotations cause the beam polarization to remain unchanged throughout the entire relaxation process. A polarization-dependent routing element (e.g., a polarized beamsplitter or birefringent element) routes the beam exiting the liquid crystal cells along either of two alternative optical paths based on the beam""s polarization.
These and other advantages, features, and objects of the present invention will be more readily understood in view of the following detailed description and the drawings.